server: Fix frame drops in SteamVR/Windows

[0forks]

The named pipe thread currently has a 5ms sleep, which leads to inconsistent tracker position updates (drops about half of the frames). This PR improves message timing consistency for OpenVR trackers on Windows by waiting for a data ready event, and runs an update as soon as the HMD position frame is received from the driver.

Comparison before and after:

frametimes.webm

[kitlith]

Finally got around to running https://github.com/kitlith/SlimeVR-Latency with this PR merged, which caused some questions, which caused me to run a profiler to check my theory.

This PR isn't setting up the async ReadFile correctly. It's blocking there. This means that the thread doesn't wake until the next time the driver sends a packet. Since this thread is waiting for skeleton calculations to be done by the server, this means that we are persistently a frame behind. The comments from people about this being much better might be because we are consistently a frame behind, instead of fluctuating between 0 and 2, but it could be improved.

When running SlimeVR-Latency, which only sends an update every second, this... takes a second to update, since it's waiting for the driver to send something else.

[0forks]

Thank you for pointing this out. This will be fixed by #629.